1. Field of the Invention
In general, the present invention relates to a coding apparatus and a decoding apparatus. More particularly, the present invention relates to a coding apparatus and a decoding apparatus wherein equalization suitable for a transmission line such as partial-response equalization is implemented, and in an operation to decode (detect) a train of codes by combining the equalization with maximum-likelihood decoding, coding is carried out by using only codes that increase a minimum intercede distance and decoding serving as a counterpart process of the coding is performed to construct a noise-proof and distortion-proof transmission system so that the size of the circuit can also be shrunk as well.
2. Description of the Prior Art
There are some digital communication apparatuses and some digital recording apparatuses wherein equalization suitable for a transmission line such as partial-response (PR) equalization is carried out and, by combining the equalization with maximum-likelihood decoding, data is detected.
In a transmission system such as one adopting a PRML (Partial Response Maximum-likelihood), for example, equalization is carried out to make a sampling waveform become convolution of codes and the codes are detected by adopting the maximum-likelihood decoding method so as to implement noise-proof and distortion-proof detection of data.
Such a data detection method has been proposed by Roy D. Cideciyan et al. as described in a document like xe2x80x98A PRML System for Digital Magnetic Recording,xe2x80x99 IEEE Journal on selected areas in communications, Vol. 10, No. 1, January 1992. In this data detection method, partial-response 4 (PR4) equalization and a maximum-likelihood decoding technique are adopted.
A digital magnetic disc recording/reproducing apparatus 10 adopting this data detection method is shown in FIG. 1. In the digital magnetic disc recording/reproducing apparatus 10 shown in the figure, data to be recorded coming from an input terminal 12 (typically, 16-bit data) is converted into 18-bit data by an encoder 13. A train of codes output by the encoder 13 with a converted bit rate is supplied to a recording head 16 by way of an amplifier 14 to be recorded into a recording medium 15. As the recording medium 15, a magnetic disc or the like can be used.
On the other hand, a train of codes played back from the recording medium 15 by a reproducing head 21 is supplied to an equalizer 23 by way of an amplifier 22. In the equalizer 23, the train of codes is subjected to partial-response equalization such as PR4 equalization. Then, an output waveform completing the PR4 equalization is supplied to a PLL circuit 24. In the PLL circuit 24, a channel clock signal CK is extracted from the output waveform and supplied to a sampler (sampling circuit) 25 for sampling the waveform completing the equalization.
A signal output by the sampler 25 is supplied to a Bitabi detector 40 to be subjected to maximum-likelihood decoding. Codes completing the maximum-likelihood decoding are then supplied to a decoder 26 for performing decoding, that is, reversed processing of the encoder 13. The decoder 26 decodes the codes to produce back the input data train which is finally output to an output terminal 27.
In such a data transmission system, as a technique of carrying out noise-proof and distortion proof detection of data, there has been proposed a method whereby partial-response equalization is further carried out as it is, and before the data is recorded, data modulation known as MTR (Maximum Transition Run) coding is performed. The MTR coding is also referred to hereafter as Trellis coding.
By adopting this technique, an intercode distance (Euclid distance) can be increased, allowing noise-proof and distortion-proof detection to be carried out. As Trellis coding, MSN (Matched Spectral Null) Trellis coding using MSN codes and time-varying Trellis coding are known.
The MSN Trellis coding was proposed by L. Fredrickson et al. as described in a document such as xe2x80x98Improved Trellis Coding for Partial-response Channels,xe2x80x99 IEEE Transaction on Magnetics, Vol. 31, No. 2, March 1995.
The time-varying Trellis coding was proposed by William G. Bliss as described in a document with a title of xe2x80x98An 8/9 Rate Time-varying Trellis Code for High Density Magnetic Recording,xe2x80x99 IEEE 1997 Digest of Intermag ""97.
By the way, the MSN Trellis coding described above and the method of detection thereof have a problem that, in order to provide the code with information on a state, the size of the circuit becomes larger than the Bitabi detection circuit composing the ordinary PRML transmission system.
In order to solve this problem, the time-varying Trellis coding described above is adopted. In the time-varying Trellis coding and the detection (decoding) thereof, however, Bitabi detection is carried out by using a 3-channel (3-time) clock signal, time control for the Bitabi detection is cumbersome and, in addition, the code maximum inversion interval [corresponding to (k+1) of a (d, k) restriction in the (d, k : m, n, r) code] becomes longer, making an overwrite operation difficult to perform accordingly. Moreover, the constraint length also increases, giving rise to a problem that a larger memory size is required for the encoder 13 and the decoder 26.
It is thus an object of the present invention addressing the problems described above to provide a coding apparatus and a decoding apparatus wherein the time control is simple, the number of codes that can be used (selected) in the encoder is increased and the maximum inversion interval as well as the constraint length are shortened without reducing the coding rate.
As a result, it is possible to provide a coding apparatus and a decoding apparatus which are suitable for a transmission system such as a digital recording/reproducing system as well as proof against noise and distortion and does not entail an increase in circuit size.
In order to solve the problems described above, the present invention provides a coding apparatus according to first aspect of the present invention characterized in that said coding apparatus has an encoder for coding an input data train to produce a train of codes comprising an even number of codes as a result of restricting 3 consecutive transiting consistent codes starting at an odd-numbered or even-numbered clock pulse of a channel clock signal and restricting all 4 consecutive transiting consistent codes.
In addition, according to third aspect of the present invention, there is provided a decoding apparatus characterized in that said decoding apparatus is used for carrying out a process of decoding a train of codes obtained as a result of coding by restricting 3 consecutive transiting consistent codes starting at an odd-numbered or even-numbered clock pulse of a channel clock signal and restricting all 4 consecutive transiting consistent codes, said decoding apparatus having a Bitabi detector whereby, after completing partial-response equalization in said decoding process, a reproduced train of codes is subjected to maximum-likelihood decoding and Trellis path time restriction is implemented during said maximum-likelihood decoding.
Moreover, according to sixth aspect of the present invention, there is provided a coding apparatus characterized in that said coding apparatus is used for producing a train of codes with an odd constraint length of at least 3 where said constraint length is the number of bits in said train of codes wherein said train of codes does not include 3 consecutively transiting consistent codes starting with an odd-numbered code, any 4 consecutively transiting consistent codes and codes starting with 2 consecutively transiting consistent codes.
Furthermore, according to eighth aspect of the present invention, there is provided a decoding apparatus characterized in that said decoding apparatus is used for carrying out a process of decoding a train of codes obtained as a result of coding carried out so that said train of codes has an odd constraint length of at least 3 where said constraint length is the number of bits in said train of codes and said train of codes does not include 3 consecutively transiting consistent codes starting with an odd-numbered code, any 4 consecutively transiting consistent codes and codes starting with 2 consecutively transiting consistent codes, said decoding apparatus having a Bitabi detector whereby, after completing partial-response equalization in said decoding process, a reproduced train of codes is subjected to maximum-likelihood decoding and Trellis path time restriction is implemented during said maximum-likelihood decoding.
According to the present invention, in implementing a time-varying Trellis coding process which allows the size of the circuit to be reduced, for an even time constraint, 3 consecutive transiting consistent codes starting at an odd-numbered or even-numbered clock pulse of a channel clock signal and all 4 consecutive transiting consistent codes are restricted in order to increase the shortest intercede distance as much as possible, and a resulting train of codes is treated as data to be recorded, that is, a train of codes to be recorded.
For an odd time constraint, on the other hand, an input train of codes is decoded while imposing restrictions thereon so that a resulting train of codes does not include 3 consecutively transiting consistent codes starting with an odd-numbered code, any 4 consecutively transiting consistent codes and codes starting with 2 consecutively transiting consistent codes.
As the partial-response equalization, the E2PR4 (Extended Extended Partial Response Class 4) equalization technique or the E3PR4 (Extended Extended Extended Partial Response Class 4) equalization technique is adopted. A train of codes is detected by adopting a combination of the PR equalization and maximum-likelihood decoding.